In a projection TV, color images are projected on a large screen by magnifying, projecting and synthesizing images from three monochrome CRTs (cathode ray tube) for B (blue), G (green), and R (red) through a projection lens. Recently, while demand for big-screen TVs rises even in ordinary homes, demand for projection TVs has also increased. There has been demand for a projection TV set that has an image quality equivalent to that of a television and is compact.
Thus, a projection lens having a high aperture ratio, a wide angle and excellent imaging performance has been required. Conventionally, these lenses have been formed only of glass or formed only of plastic in order to reduce manufacturing costs. However, recently, a hybrid lens in which glass lenses and plastic lenses having aspheric surfaces are coupled has been widely used as a lens that can be manufactured at a low cost and satisfies the conditions mentioned above.
In order to improve contrast, the lens nearest to the CRT side mainly for correcting field curvature and a CRT faceplate are coupled optically in many structures. In this structure, a thin lens (a shell lens) having uniform thickness is used in most cases, so that the lens functions as a strong concave lens by optically coupling the CRT faceplate and the shell lens using a liquid or the like. On the other hand, in order to obtain excellently focused images from center to periphery, it is necessary to correct sufficiently the optical aberration of field curvature. In a recent compact TV set, a projection lens having a wide angle with a half angle of view of more than 35 degrees also is used. Since the aberration of field curvature becomes greater as the lens has a wider angle, it is very difficult to correct the aberration only by a projection lens. In this case, as a method for reducing the burden of correcting the aberration of field curvature by a projection lens, there is a method using CRTs in which the fluorescent screen of a CRT faceplate has a concave surface on the screen side. In a recent compact TV set, this method is generally applied.
Since the performance of CRTs, lenses, screens, electric circuits and the like has been improved, the image quality of a recent projection TV has been closely approaching that of a television with a direct viewing tube. In addition, as to compactness, a projection TV is rather smaller in depth than a television with a direct viewing tube when both have the same screen size. The projection TV is inferior to the television with a direct viewing tube in the stability of image quality. Particularly, there is a difference between the image quality when turning on the projection TV and that after using it for many hours. In the case where the optical focus on a screen is adjusted to be optimum when the projection TV is turned on, image quality is deteriorated after using the projection TV for many hours since the optical focus on a screen is shifted.
The reason for the optical focal shift due to temperature change is that the coefficients of linear expansion of plastic materials used in plastic lenses and a liquid (an optical coupling liquid) used for the optical coupling mentioned above are greater than that of an optical glass, and therefore the refractive index fluctuation with respect to the temperature change is great. Particularly, as the temperature of a CRT faceplate increases after turning on the projection TV, the temperature of the optical coupling liquid contacting with the faceplate and a shell lens contacting with the optical coupling liquid increases. Consequently, the focal length of a concave lens that is formed of a shell lens and an optical coupling liquid becomes long, and the total focal length and back focus of the all lenses become short. Thus, the optical focus is shifted and therefore the image quality is deteriorated.
As a method for solving the problem described above, there is a method in which the back focus is corrected so as not to become short by shifting the shell lens toward the screen side as temperature increases. Although patent applications related to this method have been filed, the method has not been put into practical use widely. The reason is that a mechanical structure that shifts the shell lens suitably in the direction of an optical axis without giving the shell lens any tilt is required, thus increasing the costs.